1. Field of the Invention
The present application relates generally to systems and method for reducing error contributions to gyroscopic measurements from a wellbore survey system and/or determining the position or orientation of the survey system relative to the Earth.
2. Description of the Related Art
Many wellbore gyroscopic survey systems that are currently in service are based on angular rate measurements taken about two axes only, denoted the x and y axes, that are both substantially perpendicular to the direction along the wellbore (referred to as the “along-hole axis”) and substantially perpendicular to each other. In stationary gyroscopic survey systems, these measurements are used to determine the direction of the survey tool in the wellbore with respect to true north, the tool azimuth angle, using measurements of the horizontal components of Earth's rotation sensed about a measurement axis of the survey tool in a process known as gyro compassing or north finding. In many such systems, the gyroscopes (“gyros”), and other inertial sensors (e.g., accelerometers) used by the survey system, are attached rigidly or via anti-vibration mounts to the housing of the survey tool in what is referred to as a strapdown mechanization.
In many such survey tools, it is common practice to take two sets of gyroscopic sensor measurements of the Earth's angular rotational rate in two different directions substantially perpendicular to the along-hole direction, typically by rotating the xy-gyros through 180 degrees about the along-hole axis of the survey tool between each set of readings. This procedure is referred to as “indexing” the gyro, and it yields substantial benefits in terms of both the speed with which tool direction with respect to true north can be determined and the accuracy to which that direction can be obtained. The latter benefit derives from the fact that the effect of gyro measurement biases can be substantially reduced, or removed completely, through indexing the gyro.
The indexing of the xy-gyro can be achieved by mounting this sensor on a rotatable platform that can be turned between the two index positions that are usually 180 degrees apart. Such a configuration is disclosed in U.S. Pat. Nos. 5,657,547 and 5,806,195, each of which is incorporated in its entirety by reference herein. Upon the turning of the xy-gyro, the components of Earth's rotation sensed by the xy-gyro change sign between the two index positions at which the readings are taken, but the signs of any residual biases do not change. Hence, by summing the two measurements from the xy-gyro and dividing the result by two, an estimate of the residual bias is obtained. Similarly, by calculating the difference between the two measurements and dividing the result by two, an improved estimate of the true applied rotation rate can be extracted that is not corrupted by any fixed bias in the gyro measurements. Given knowledge of the inclination and tool face angle of the tool, derived from accelerometer measurements, together with knowledge of the true rotation rate of the Earth and the latitude at which the measurements are being taken, an estimate of the azimuth angle of the survey tool may be obtained. While azimuth can be determined using a strapdown system, the process takes considerably longer to implement without the facility to index the gyro.
Indexed gyro compassing may be achieved with a single gyro by mounting the gyro and its indexing mechanism on stable platform within the survey tool so as to maintain the index axis coincident with the local vertical. In theory, such a system could be used to determine the direction of the survey tool with respect to true north, irrespective of tool orientation. However, the mechanical complexity and consequent size of such a system preclude it as a viable option for down-hole application.